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Sunday, October 22, 2017

Wood gas engine power loss

Posted by Krzysztof Lis on April 4, 2008

If you use wood gas in your internal combustion engine, designed for gasoline or diesel fuel, you’ll notice some power loss. In this article I’ll describe why such power loss does occur and how to overcome this effect.

First of all, it needs to be stated, that wood gas has very low heating value. For wood gas that consists of 17-22% (by volume) carbon monoxide, 12-20% of hydrogen and 2-3% of methane, the heating value is only 5,000-5,900 kJ/m³. When you mix wood gas with air in ratio close to 1:1,1 (stoichiometric mixture), its heating value is around 2,500 kJ/m³. Stoichiometric mixture of gasoline with air has heating value of 3,800 kJ/m³ [1]. Because of that, less energy is introduced to the engine, and less power is generated by the engine.

In most cases, this power loss can be up to 30-40%. But hey, if you want to have your car fueled by wood gas, you probably found some other good reason to build your gasifier. Nevertheless, you’ll probably interested in overcoming this power loss.

We’ll use the great advantage of wood gas – the high octane rating. This rating describes how good does fuel work in spark ignition engines, i.e. when does it start to knock (it’s a measure of anti-detonation of fuel). If a fuel has high octane rating, it can be used in engines with high compression ratio. And guess what? Wood gas has octane rating greater than gasoline!

What does it mean? While you can’t use standard gasoline in engines with compression ratio higher than something around 10:1, because you’ll have knocking (detonation combustion). And the higher the CR, the higher the engine efficiency (the more power it gives from the same amount of fuel). So if you plan on using your engine exclusively on wood gas, you might consider increasing the CR, for example by installing thinner cylinder head gasket. Increasing CR will help you overcome the wood gas power loss.

If you don’t want to increase engine CR, you can give it additional power by installing supercharger or turbocharger. This latter device increases the amount of fuel / air mixture using the kinetic energy of hot exhaust gasses from your engine.

Diesel engines don’t use stoichiometric fuel / air mixtures, but lean mixtures. The cylinder is full of air and some fuel is introduced. So total energy introduced to the engine per unit volume is closer to case when you filled the cylinder with wood gas / air stoichiometric mixture. Because of that, in diesel engines the power loss is much smaller than for gasoline engines!

[1] Wood gas as engine fuel, FAO Forestry Department, Forest Industries Division, Mechanical Wood Products Branch

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